Abstract
We investigate the viscoelastic properties of confined molecular nano-layers by time resolved optical pump-probe measurements. Access to the elastic properties is provided by the damping time of acoustic eigenmodes of thin metal films deposited on the molecular nano-layers which show a strong dependence on the molecular layer thickness and on the acoustic eigen-mode frequencies. An analytical model including the viscoelastic properties of the molecular layer allows us to obtain the longitudinal sound velocity as well as the acoustic absorption coefficient of the layer. Our experiments and theoretical analysis indicate for the first time that the molecular nano-layers are much more viscous than elastic in the investigated frequency range from 50 to 120 GHz and thus show pronounced acoustic absorption. The longitudinal acoustic wavenumber has nearly equal real and imaginary parts, both increasing proportional to the square root of the frequency. Thus, both acoustic velocity and acoustic absorption are proportional to the square root of frequency and the propagation of compressional/dilatational acoustic waves in the investigated nano-layers is of the diffusional type, similar to the propagation of shear waves in viscous liquids and thermal waves in solids.
Highlights
Damping time with the molecular layer thickness is modelled analytically including the viscoelastic properties of the molecular layer
An optical pulse excites the coherent dynamics in the sample, which are subsequently probed by measuring the reflectivity of a second, weaker and time delayed optical pulse
Visible is a striking dependence of the obtained damping times on the thickness of the molecular layer, which acts as a barrier for the coherent phonons on their way from the gold film generator to the substrate
Summary
Damping time with the molecular layer thickness is modelled analytically including the viscoelastic properties of the molecular layer. We choose aminopropyltrichlorosilane (APTES) as the organic interface layer due to its wide use in nanotechnology as adhesion promoter[10,11] This type of molecule allows to grow films down to monolayer thickness via self-assembly and the amino group of the molecule hinders gold diffusion into the molecular layer. An optical pulse excites the coherent dynamics in the sample, which are subsequently probed by measuring the reflectivity of a second, weaker and time delayed optical pulse. This yields the time resolved optical response of the sample including the temporal evolution of the excited coherent longitudinal (compression/dilatation) phonons. The electrons thermalize via electron-electron and electron-phonon interaction where the latter causes an impulsive heating of the gold film and excites a coherent vibrational mode in the gold film by the thermoelastic process
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